Potassium chloride, potash or sylvite is mainly used as a fertilizer, either as single fertilizer or in combination with other plant nutrients. Potassium chloride is also used in several industries like dyes, soaps, detergents, food, pharmaceuticals and as starting material for preparing other potassium chemicals like potassium hydroxide and potassium carbonate.
When sea water is evaporated, various sea salts precipitate. Generally, the order of precipitation of sea salts is as follows: halite or sodium chloride (NaCl), epsomite (MgSO4.7H2O), kainite (MgSO4.KCl.3H2O), and carnallite (MgCl2.KCl.6H2O)— epsomite (MgSO4.7H2O) mixture. Sodium chloride is mixed in all precipitated salts in various quantities. Potassium chloride does not precipitate from sea water. Potassium chloride is recovered from sulfate free terminal lakes like the Dead Sea and Bonneville Lake or is mined either by solution mining or by regular mining. In order to obtain potassium chloride from sea water, sulfates need to be removed from the water.
The removal of sulfate can be achieved by refrigeration or cooling of the sea water bitterns to subzero degrees to remove epsomite, MgSO4.7H2O. The low sulfate bitterns is then evaporated to provide carnallite that can be processed to obtain potassium chloride. Refrigeration cannot remove all sulfates. Thus, potassium chloride obtained by this process is contaminated with sulfates to some extent.
In order to achieve complete removal of sulfates, use of calcium chloride has been tried by several researchers. Calcium chloride reacts with epsomite to precipitate gypsum, CaSO4.2H2O. However, this is an expensive process.
Several attempts have been reported using carnallite to obtain potassium chloride. However, carnallite from sea water contains epsomite, and potassium chloride obtained from seawater carnallite is contaminated with epsomite.
U.S. Pat. No. 3,099,528 discloses a process to produce KCl via sylvinite obtained from crude carnallite. In this process calcium chloride is used to desulfate the brine. As discussed above, this is an expensive route.
U.S. Pat. No. 7,014,832 reports an integrated process for the simultaneous recovery of industrial grade potassium chloride and low sodium edible salt with overall KCl yield of 90 to 95%. The process comprises removal of sulfates from bitterns with a solution of calcium chloride; subjecting the desulfated bittern to evaporation; mixing the concentrated desulfated bittern with a concentrated solution of MgCl2 to obtain carnallite and further obtaining KCl. This is an expensive route due to cost of raw materials and process steps involved.
U.S. Patent Publication No. 2003/0080066 discloses an integrated process for recovery of high purity potassium chloride, and end bittern containing 7.5 g/L bromine. The process is based on desulftation of brine with distiller waste of soda ash industry or calcium chloride generated from limestone and acid. The process has limited applicability due to requirement of distiller waste and the carnallite obtained is contaminated with high extent of NaCl necessitating further purification.
PCT International Publication No. WO/2003/064323 discloses a method of producing KCl-enriched low sodium salt from bittern. This process also uses calcium chloride and is expensive.
US 2005/0220697 and WO/2005/063625 describe treatment of carnallite decomposition liquor (CDL) with lime (CaO) to obtain KCl. The publication describes that CDL is obtained by processing bittern by following the steps described.
BR 8107507 described recovery of alkali metal halides, alkaline earth metal halides, and Mg(OH)2 from final brines (the mother liquor remaining after the 1st crop of NaCl from seawater). The process involves NaCl and CaCO4 removal by evaporation and crystallization, Ca(OH)2 addition to permit Mg precipitation and removal as insoluble Mg(OH)2, brine reconcentration, and removal and separation of a mixture of NaCl and KCl crystals. The process is carried out in a system of agitators, filters, evaporators, grinders, flotation cells, and thickeners.
DE 2219340 described that mother liquor of salt evaporation pond is reacted with calcium chloride for the purpose of desulfating and then evaporated to precipitate sodium chloride and carnallite. The carnallite is subjected to a cold decomposition process with water to obtain potassium chloride.
SE 95638 describes processes for extracting NaCl from sea water as well as, other substances in their original or changed forms. The processes involves concentration of the sea water by freezing the water to form ice crystals that are separated from the solution gradually. The concentrated solution is further processes to obtain sodium chloride and other salts.
Although, there have been many attempts reported in the literature to obtain potassium chloride compositions from sea water, there continues to be a need for cost-effective and efficient processes for production of compositions comprising potassium chloride and sylvinite from sea water, bitterns and sea salts.